The present invention relates to an optical one-to-one protection switching apparatus, more particularly to an optical one-to-one protection switching apparatus employed to prevent miss-connection that might occur between a transmitter and a receiver while a working line and a protection line are switched.
Optical Networks (Ramaswami et al. Morgan Kaufman publishers pp430-434) is a known example in this field.
FIG. 12A shows a block diagram of an optical one-to-one protection switching apparatus in the related art. In the case of this related art, extra traffic can be included. According to this optical one-to-one protection switching apparatus, client terminals 200 and 210 at different points communicate with each other through first transmission lines 180 and 185. At this time, the optical switches 145 and 165 select the first transmission line 180 for down-stream communications. For up-stream communications, the optical switches 175 and 155 select the first transmission line 185. The client terminals 205 and 215 communicate with each other through the second transmission lines 190 and 195. At this time, the optical switches 145 and 165 select the second transmission line 190 for down-stream communications and the optical switches 175 and 155 select the second transmission line 195 for up-stream communications. The arrows in each optical switch denote a transmission direction of signals. Solid lines arrow denote “the normal state”, and broken lines denote “a state after switching”. The same notations are used in the subsequent embodiments.
Referring to FIG. 12B, if a fault is detected in the first transmission line 180 or 185 in down-stream or up-stream communications, the optical switches are changed between 145 and 155, as well as between 165 and 175. Thereby, the client terminals 200 and 210 communicate with each other through the second transmission lines 190 and 195. The client terminal 205 or 215 that transmits low priority extra traffics communicates with each other through the first transmission line 180 or 185.
Unlike the configuration of the optical one plus one protection switching, the configuration of the optical one-to-one protection switching requires communications of control signals between nodes. The optical one-to-one protection switching configuration is characterized by the inclusion of extra traffic. However, because the optical switch changes the corresponding transmission lines in the configuration, the following problem (A) arises. Problem (A): If any extra traffic is included in a transmission line, when the switching block uses only optical switches as shown in FIG. 12A, the configuration might experience a miss-connection between client terminals in the switching process.
FIG. 12B shows a miss-connection between client terminals in the switching process after a switching request is issued. In FIG. 12B, the miss-connection occurs during a switching operation when a fault is detected in the first transmission line 180 in the down-stream communications. Still also referring to FIG. 12A, in the normal state, the transmitter 200a of the client terminal 200 and the receiver 210b of the client terminal 210 are connected to each other through the first down-stream transmission line 180. The transmitter 205a of the client terminal 205 and the receiver 215b of the client terminal 215 are connected to each other through the down-stream second transmission line 190.
In the case of a fault/maintenance, the optical switches 145 and 155 are switched in the transmit node 120. During this switching in the transmit node 120, their connection states are changed, and a miss-connection might arise. Concretely, the transmitter 200a of the client terminal 200 and the receiver 215b of the client terminal 215 are connected to each other through the down-stream transmission line 190 as partially indicated by dotted-lines. The transmitter 205a of the client terminal 205 and the receiver 210b of the client terminal 210 are connected to each other through the first down-stream transmission line 180 as partially indicated by dotted-lines. After the switching is ended, the connection is normalized after both transmit node 120 and receive node 130 are changed over.